The invention concerns a method for estimating motionin a television signal between first and second video images. The purpose of estimating motion in a television signal is to reduce the amount of data transmitted. This reduces the required transmission bandwidth, transmission time and video recorder storage capacity. In accordance with the motion estimate, successive pictures are not transmitted completely as in the case of a conventional television signal. Instead, difference factors are transmitted which represent any alteration between successive pictures. A further usage of the motion estimate occurs in applications involving switching-over certain signal processing circuits, e.g. comb filters, as regards their manner of operation in dependence on image motion in the television signal. A motion estimate is also required in video cameras to electronically or mechanically balance out alterations which occur in the image due to unintentional shaking of the camera.
The greater the search range, the more effective is the estimate of the motion between video images. In order to reduce the amount of hardware which is associated with searches, a hierarchical search arrangement is adopted. Initially and in a relatively coarse manner, a best motion factor is sought throughout the whole search range. To improve this factor, a more finely resolved search is performed in the neighborhood of the location that has been found.
In the known "block matching algorithm" method of motion estimation, a video image that is to be processed is divided into blocks of fixed size. The motion with respect to a second video image is determined for each block in that an error magnitude (distortion) of the block is calculated for each possible displacement within a search range, and thereafter the minimum one of the values is determined.
The implementation in hardware is based on commercially available motion estimation integrated circuits such as the STI3220 device type commercially available from SGS-Thomson for example. This module computes the displacement having the least distortion in a search range of -8/+7 points (pixels) horizontally and vertically. The search range can be enlarged by cascading a plurality of integrated circuits. For high resolution HDTV systems, and for video images in accordance with the MPEG standard in which motion between images lying further apart in time is computed, as large a search range as possible is desired. This is to estimate the motion and determine displacements with a resolution of 1/2 pel (pel =pixel =picture element). With "full search block matching", in which each possible displacement requires a parallel arithmetic unit, several hundred modules may be required even when using the integrated motion estimation chips.